We demonstrated all-optical demultiplexing of 160-Gb/s signal to 40- and 80-Gb/s by a Mach-Zehnder Interferometric all-optical switch, where the picosecond cross-phase modulation (XPM) induced by intersubband excitation in InGaAs/AlAsSb coupled double quantum wells is utilized. A bi-directional pump configuration, i.e., two control pulses are injected from both sides of a waveguide chip simultaneously, increases a nonlinear phase shift twice in comparison with injection of single pump beam with forward- and backward direction. The bi-directional pump configuration is the effective way to avoid damaging waveguide facets in the case where high optical power of control pulse is necessary to be injected for optical gating at repetition rate of 40/80 GHz. Bit error rate (BER) measurements on 40-Gb/s demultiplexed signal show that the power penalty is decreased slightly for the bi-directional pump case in the BER range less than 10-6. The power penalty is 1.3 dB at BER of 10 - 9 for the bi-directional pump case, while it increases by 0.3-0.6 dB for single pump cases. A power penalty is influenced mainly by signal attenuation at "off" state due to the insufficient nonlinear phase shift, upper limit of which is constrained by the current low XPM efficiency of 0.1 rad/pJ and the damage threshold power of 100 mW in a waveguide facet.

A one-dimensional finite-difference time-domain (FDTD) simulator for ultrafast optical switches based on intersubband transition (ISBT) in GaN/AlN waveguide is described. Influences of the inhomogeneous broadening and the 2D mode profile have been taken into consideration. The ultrafast optical response (τ 185 fs) measured in a GaN/AlN waveguide was successfully reproduced by the simulator. At present, however, the saturation characteristics of the fabricated device are mainly limited by the excess TM loss caused by the dislocation in MBE-grown nitride layers. When the dislocation density is reduced and the structure is optimized, the switching pulse energy will be improved to about 10 pJ. Further reduction ( 1 pJ) will be possible when low-loss submicron waveguides with spot-size converters are developed.

In this report we present all-optical switches and modulators based on the intersubband transition in semiconductor quantum wells. The use of InGaAs/AlAsSb coupled double quantum well structures is proposed to facilitate intersubband transitions in the optical-communication band, and to reduce the intersubband absorption recovery time from several picoseconds to a few hundred femtoseconds by utilizing enhanced electron-phonon scattering. Subpicosecond all-optical gating and modulation in coupled double quantum wells are observed using pump-probe experiments at optical-communication wavelengths. The results indicate that the intersubband transition in this structure is very useful for ultrafast all-optical switching devices.

Simultaneous wavelength conversion utilizing four-wave mixing in optically-pumped GaN/AlN intersubband optical amplifiers has been investigated by means of a finite-difference time-domain (FDTD) model. The conversion efficiencies at a pump power of +7-+10 dBm were predicted to be -9-+6 dB depending on the frequency detuning (0.3-10.9 THz). The difference in efficiency among 18 channels of WDM signals with 100-GHz spacing was within about 3 dB.

Although all-optical gate switches based on the intersubband absorption in nitride quantum wells are predicted to operate at 1 Tb/s, realization of strong intersubband absorption at the optical communication wavelength is still difficult. An alternative approach is an interferometer-type gate switch utilizing refractive index change due to the intersubband absorption of a control pulse at a longer wavelength. Feasibility of Mach-Zehnder interferometer (MZI) gate switches, in which 1.55-µm pulses are controlled by 1.85-µm pulses, was theoretically investigated by finite-difference time-domain (FDTD) simulator. Although the effective phase shift does not reach π, 22.5% of the signal pulse energy was predicted to be gated by a 10-pJ control pulse in the MZI switch. Even 1.3-µm pulses can be controlled by 1.85-µm pulses at the expense of the switching energy. This approach provides a way to process signal pulses at the optical communication wavelength utilizing strong intersubband absorption at a longer wavelength.

The propagation and the gate operation of femtosecond pulses in nonlinear optical waveguides utilizing the saturation of the intersubband absorption at 1.55 µm in nitride multiple quantum wells are simulated for the first time. The calculation was carried out by a one-dimensional finite-difference time-domain (FD-TD) method combined with three-level rate equations describing the intersubband carrier dynamics. The absorption recovers within 1 ps when the pulse width is less than 200 fs, which will allow 1-Tb/s operation. However, the pulse shape may be deformed with the propagation due to the coherent effect and the interference between the signal and the control pulses, and thus, optimization of the pulse widths and the incident timing is required. Since the transparent window (width of the control pulse) becomes shorter according to the propagation, the width of the control pulse should be set broader than that of the signal pulse. As an example, we assume the case where a 1.6-µm, 100-fs signal pulse is gated by a 300-fs control pulse at a wavelength of 1.5 µm in a 500-µm length waveguide. A 140-fs gated signal pulse with a smooth envelope is expected to appear after the band-pass filter. The extinction ratio is expected to be greater than 15 dB.